1. Field of the Invention
This invention generally relates to an electrophotographic method or process and an apparatus therefor. More particularly, it relates to a novel electrophotographic process which is less affected by environmental conditions during image reproduction than the conventional electrophotographic processes and can produce copy images of very high sensitivity and very high definition.
2. Description of the Prior Art
Applicant previously proposed various electrophotographic processes as disclosed in U.S. Pat. No. 3,438,706, U.S. Pat. No. 3,666,363, U.S. Pat. No. 3,666,365 and copending U.S. Application Ser. No. 563,899. These electrophotographic processes employed a photosensitive medium basically comprising a conductive back-up member, a photoconductive layer and an insulative layer, and involved the steps of pre-charging the surface of the outermost insulative layer of the photosensitive medium with positive polarity if the photoconductive layer is of N-type conductivity or with negative polarity if this layer is of P-type conductivity so that charges opposite in polarity to the charges in the surface of said insulative layer may be formed in the region adjacent the interface between the photoconductive layer and the insulative layer; thereafter applying light from an original image and AC corona discharge or DC corona discharge of the opposite polarity to that of the pre-charge simultaneously to the surface of the pre-charged insulative layer to thereby vary the charged condition of the photosensitive medium in accordance with the light-and-dark pattern of the original image; and then substantially uniformly exposing the entire surface of the insulative layer to light to thereby release the charges corresponding to the dark region of the original image which exist in the vicinity of the interface between the photoconductive layer and the insulative layer, thus forming on the surface of the photosensitive medium an electrostatic latent image of high contrast representing the original image. Further, such electrostatic latent image is developed into a visible image by means of a developer composed chiefly of charged toner particles, whereafter the visible image is transferred to paper or other transfer medium by utilization of an internal or an external field, and then the transferred image is heated and fixed by an infrared ray lamp or the like to provide an electrophotographic copy image. On the other hand, after the image transfer, the surface of the insulative layer of the photosensitive medium is cleaned to remove the residual charged toner particles therefrom so as to make the photosensitive medium ready for reuse.
In addition to such conventional processes, there are known further electrophotographic processes including that which employs a photosensitive layer of P.I.P. characteristic or which involves the step of simultaneous application of image light and discharge or charge.
In these electrophotographic processes, corona discharge was generally used as the means for discharging or charging the surface of the photosensitive medium substantially simultaneously with the application of original image light thereto to thereby form an electrostatic latent image. Such processes using the corona discharge are meritorious in that the means therefor is not in contact with the photosensitive medium, whereas they have numerous demerits that the corona discharge electrode is readily subject to contamination which in turn results in an irregular discharging or charging effect, that the intensity of the corona discharge is fluctuated with variations in the environmental conditions such as temperature, humidity, atomospheric pressure, etc., that the shadow of the corona discharge electrode may be projected on the photosensitive medium depending on the construction of the optical system in use, and that the use of a high voltage may cause dangers. Also, using corona discharge or charge during the simultaneous application of image light and discharge to speed up the above-described various electrophotographic processes would result in a limited discharging or charging effect because corona discharge has a relatively high discharge resistance, and if any means for increasing the applied voltage is adopted to realize the speed-up, the resultant electrostatic latent image may often have a reduced contrast. In the previous electrophotographic processes, the photoconductive layer is advantageously required to maintain its charge only during the discharging time, so that a photoconductive material of low resistance can be used to obtain a very high sensitivity, nevertheless the corona discharge employed for the discharging or charging to be effected simultaneously with the application of image light imposed limitations in reducing the discharging or charging time, and accordingly imposed limitations on the resistance of the available photoconductive material.
Further, where an electrostatic latent image is formed by using corona discharge to effect discharging or charging substantially simultaneously with the exposure to image light, the potential in the light region of the latent image is limited to the vicinity of the saturation potential of the corona charger in order to provide a sufficient contrast of the latent image. The saturation potential of a corona discharger is usually zero to several hundred volts of negative sign in case of AC corona discharge, and one thousand and several hundred volts to two thousand and several hundred volts in case of DC discharge, and any other saturation potential than these ranges could not be provided without using a control electrode or without using an AC corona discharge with a DC bias imparted thereto. AC corona discharge is generally employed for the purpose of discharging, but the use of AC corona discharge in a frequency range of 50 to 60 cycles would readily cause irregular discharging effect in these cycles.
In addition to the discharging or charging method which employs corona discharge, a method using an electrode has heretofore been proposed, whereas this method is very difficult to realize because the allowance for the clearance between the electrode and the photosensitive medium is very much limited. An excessive clearance would lead to irregular discharging effect and therefore, it is hardly possible especially to discharge a photosensitive medium in motion.
As has been noted above, the use of corona discharge during the step of simultaneous application of image light and discharging or charging in the electrophotographic processes including such step involves numerous disadvantages and problems which ought to be overcome.